Amounts of data to be stored continue to increase. Data storage apparatus can range from various forms of electronic memory to disk drives, such as RAID (redundant array of independent disks), JBOD (just a bunch of disks), to optical disk and magnetic tape drives and libraries. Data storage can be for a single host or for an entire network, for example, through Network Attached Storage (NAS).
Typically, data is stored on apparatus in a fashion related to the need to retrieve the data. For example, data comprising or used by an application of a host computer processor system is stored in apparatus offering higher speed access, such as electronic memory or disk drives. Data not likely to be accessed is typically stored to optical disk or magnetic tape. The optical disk or magnetic tape may also be subject to removal from the system and archived.
Generally speaking, higher speed data storage apparatus is more expensive per amount of data stored than is the lower speed data storage. Thus, in the typical data processing system, a smaller amount of higher speed data storage is provided, and the bulk of data is stored in the lower speed data storage. A typical procedure is to store data in higher speed data storage so long as it is used reasonably often, and then the data is destaged to lower speed data storage such as magnetic tape, for example, using a least recently used (LRU) algorithm. Data is typically retrieved by first going to the higher speed data storage or storage server, and, if the data is not there, then subsequently staging the data back from the lower speed data storage to the higher speed data storage for access.